The present invention relates to methods for controlling static field and MRI (magnetic resonance imaging) apparatus. More specifically, the present invention relates to static field regulating methods for controlling the static field uniformity and an MRI apparatus suitably carrying out the same static field regulating methods.
FIG. 12 shows a partial cross-sectional view of a prior art MRI apparatus.
The MRI apparatus 500 is an open-type MRI apparatus which generates a static field Bo in the vertical direction.
A static field generator magnet is constituted of permanent magnets 1M1, 1M2 oppositely placed at top and bottom, a base yoke YB, columnar yokes Yp, and deflectors Sp1.
The permanent magnets 1M1 and 1M2 and the base yoke YB may incorporate screws 51 of magnetic material for static field regulation. The amount of threading of the screws 51 may enable adjusting the uniformity of the static field Bo in the imaging area KK.
In the MRI apparatus 500 as have been described above, there is first primary problem that the permanent magnets 1M1 and 1M2 and base yoke YB and deflector Sp1 may be susceptible to change in temperature, after the amount of threading of the screws 51 has been adjusted. For example, when using Nd—Fe—B material for the permanent magnets 1M1 and 1M2, which material has its temperature characteristics of approximately −0.1% /° C. , if the temperature of either one of the permanent magnets 1M1 and 1M2 is raised from the temperature of the other magnet after the amount of threading of the screws 51 has been adjusted, then some static field inconsistency of the significant size will occur in the static field direction (direction of Z-axis), which is not neglectable, however the system cannot address it.
Also, in such a system as the MRI apparatus 500 that may adjust the amount of threading of the screws 51, there may be second major problem that either the adjustable range or the adjusting accuracy of the uniformity of the static field Bo will be traded off. For example, if the permeability of the screws 51 is enough larger, then the adjustable range will be wider while on the other hand the adjustment accuracy will become coarse. Contrarily if the magnetic permeability is enough smaller then the adjustment accuracy will be finer while on the other hand the adjustment range will be narrowed.